1. Technical Field
The present invention relates to a toner for developing an electrostatic latent image, a developer for developing an electrostatic latent image and an image-forming process for use in electrophotography copying machines, electrophotography facsimiles, electrophotography printers, and the like.
2. Related Art
Generally, electrophotographic processes include (1) a step of charging a photoreceptor, (2) a step of forming a latent image on the photoreceptor by exposure thereof to light, (3) a step of developing the latent image with a developer including a toner, (4) a step of transferring the developed toner onto a medium such as paper, and (5) a step of fixing the toner image transferred onto the medium. Among the above steps, a heat roll method of heating the toner directly with a heated roller, and a flash-fixing method of fixing the toner by heat generated by irradiation of a flash light, such as that from a xenon, halogen, or other lamp, are well known as the fixing step.
An advantage of the heat roll method is that it is possible to easily make the surface of a fixed toner image smooth and uniform to a certain extent, even when a material that changes its viscosity by heating and shows an elastic repulsion under applied pressure, such as a binder resin contained in toner, is used, because the toner is fixed on paper as it is heated directly at high temperature under pressure by a heating roll. However, use of this method often results in fluctuation in image characteristics such as glossiness, because the heating roll is often cooled by paper while the paper is conveyed in the fixing device and thus, the fixing temperature often declines when a number of sheets of paper are printed continuously at high speed. It is necessary to raise the temperature setting of a heating roller to overcome this disadvantage, occasionally leading to a heating-roller surface temperature of 250° C. or higher, deterioration of the surface of the heating roller, and consequently to deterioration in releasing characteristics.
In contrast, the flash-fixing method has the following advantages:
1) there is no staining of images due to offsetting of the residual toner that is generated when heating rollers are used, because it is a non-contact fixing process;
2) it does not demand a design for preventing the image staining often found during contact fixing (addition of a releasing agent to toner, or coating of a fixing roller with a releasing agent such as silicon oil), and thus, there is a greater freedom in designing the toner and the fixing unit; and
3) it is possible to fix the images irrespective of the material and the thickness of the recording paper.
For these reasons, the flash-fixing method may be used in high-speed applications such as high-speed printers and high-speed copying machines. However, the flash-fixing method raises the surface temperature of toner instantaneously to 300° C. or higher.
Components contained in a toner sublime at such a high temperature, occasionally resulting in internal contamination of a device.
Thus, it is necessary to adopt measures to prevent such troubles, such as a method of reducing the amount of the sublimates by controlling the amount of low-molecular weight components in the binder resin, which is a component of the toner (e.g., Japanese Patent Application Laid-Open (JP-A) No. 2001-117271), or a method of installing a dust-collecting mechanism such as an exhaust gas filter (e.g., JP-A Nos. 7-219381 and 2000-250368). However, among the sublimates, more polar binder resin-derived components decompose and sublime during flash fixation at high temperature, but other less-polar materials remain un-decomposed and low polarity sublimates cool rapidly and deposit on the exhaust gas filter or other members. As a result, low polarity sublimates cool and harden at the upper layer of the filter and are thus deposited on the surface of each member and when, for example, an exhaust gas filter is used, this leads to clogging of the filter, causing problems such as inefficient use of the filter surface and shortening of the lifetime of the filter.